In recent years, a solar cell, that is, a device that directly converts solar energy into electric energy has become widely used. Such solar cells provide an effective use of natural resources and prevent environmental pollution. Further development of solar cells is in progress.
As shown in FIG. 1, a solar cell generally includes a front side transparent protection member 11, a front side sealing film 13A, photovoltaic cells 14 such as power generation elements made of silicon, a back side sealing film 13B, and a back side protection member (back cover) 12. The forgoing elements are successively superimposed in this order. After the solar cell is subjected to vacuum degassing, heat and pressure are applied to the solar cell so that the front side sealing film 13A and back side sealing film 13B are adhesively integrated with each other by crosslinking or curing the films 13A and 13B. Known solar cells are prepared by connecting a plurality of photovoltaic cells 14 with each other for generating a large electrical output. To maintain the electrical insulation property of the photovoltaic cells 14, the sealing layers 13A and 13B are provided with electrical insulation properties for sealing the photovoltaic cells 14.
Films of ethylene-polar monomer copolymer such as ethylene-vinyl acetate copolymer (EVA) and ethylene-ethyl acrylate copolymer (EEA) are used as sealing films in solar cells (Patent literature 1). The ethylene-polar monomer film is prepared by application of heat and pressure by use of rollers, to a composition comprising ethylene-polar monomer copolymer and a crosslinking agent.
Depending on the properties required for the solar cells, various fillers are used in the sealing films of the solar cells. For instance, when the back side sealing film colored by a pigment is used, light reflection occurs at an interface between the back side sealing film and the light-receipt side (front side) sealing film, or diffused light reflection occurs due to the presence of pigment in the back side sealing film. Namely, an incident light is irradiated to parts between the photovoltaic cells and light is transmitted through the cells. These lights are subjected to the diffused light reflection, and the diffused light enters the photovoltaic cells again as incident light. Accordingly, efficiency for using light, and power generation capability are improved. By use of pigments, an aesthetic or design property of the sealing film can be also improved. As the pigments, in general, various inorganic or organic pigments such as white, black, blue, red or the like are used. More specifically, for example, white pigments based on titanium oxide or calcium carbonate; blue pigments based on ultramarine; black pigments based on carbon black; and translucent pigments such as glass beads and a light scattering agent are used.    Patent Literature 1: JP-A 06 (1994)-177412